1. Field of the Invention
The present invention relates to synchronous motors to be used for various industrial devices and hybrid electric vehicles, etc., and more particularly, relates to double stator synchronous motors having a double stator structure and which consume a less amount of rare earth magnets or no rare earth magnet by using ferrite magnets instead of rare earth magnets.
2. Description of the Related Art
There are known double stator electric motors having a double stator structure composed of an outer stator, an inner stator and a rotor. In a conventional double stator electric motor, the outer stator is disposed on a radially outer side of the stator and the inner stator is disposed on a radially inner side of the rotor.
For example, a conventional patent document, Japanese laid open publication No. 2001-244643 discloses an electric motor having a rotor, an inner stator and an outer stator.
In the rotor, a plurality of segment magnetic poles is arranged in a circumferential direction of the rotor at regular intervals apart from each other. The segment magnetic poles are made of soft magnetic material. A neodymium magnet is arranged between the adjacent segment magnetic poles.
The stator has the outer stator and the inner stator. Each of the outer stator and the inner stator has magnetic poles, whose number is the same number of the segment magnetic poles in the rotor. The outer stator and the inner stator have the same magnetomotive forces, and are disposed in face-to-face relationship to each other through the segment magnetic poles of the rotor.
Because the conventional electric motor disclosed in the conventional patent document has a small magnetic resistance of a magnetic circuit and decreases an amount of magnetic flux leakage, it is possible to relax the saturation of the iron core and decrease an iron core loss, and this makes it possible to improve the efficiency of the motor at high speed rotation.
In addition, because a reluctance torque is generated when magnetic flux passes in a circumferential direction of the rotor through the segment magnetic poles, and it is thereby possible for the electric motor to generate a large output torque without using any strong magnet. This makes it possible to promote a size reduction of the electric motor and to save the consumption of rare earth magnets.
By the way, there is a strong demand for electric motors such as surface permanent magnet synchronous motors (IPMSM) to more decrease the consumption of such rare earth magnets and to use ferrite magnets instead of the use of rare earth magnets due to the depletion of rare earth resources and the increased cost of rare earth resources. It is therefore desired to progress in technical development of the electric motor field.
However, the electric motor as the conventional technique disclosed in the conventional patent document has a decreased output torque when using ferrite magnets instead of using neodymium magnets arranged between adjacent segment magnetic poles because such ferrite magnets have a low magnetic flux density which is smaller than a magnetic flux density of neodymium magnets.
In addition, because such ferrite magnets have a low magnetic field at a critical point in a demagnetization curve, demagnetization easily occurs in an electric motor having windings of a large magnetomotive force such as a drive motor used for electric motor vehicles. That is, because ferrite magnets have a low residual magnetic flux density and low coercive force, it is difficult in principle for such ferrite magnets to generate a large output torque in a limited space of an electric motor, as compared with the output torque when using neodymium magnets.
A first subject in the conventional techniques to be solved is to find a method of having a large magnet area when ferrite magnets are used instead of neodymium magnets.
A second subject in the conventional techniques to be solved is to find a method of using ferrite magnets having a low critical point in a demagnetization curve without causing demagnetization.
A third subject in the conventional technique to be solved is to find a method of effectively using a reduced amount of neodymium magnets.